Hide and disable Drupal Migrate Multilingual

Problem/Motivation

Once the multilingual migration path is fully stable (#2208401: [META] Stabilise Migrate Drupal Multilingual module ), we have no need for the separate Migrate Drupal Multilingual module.

Proposed resolution

Hide the module from the UI, disable it and let migrate discover multilingual migrations normally.

Remaining tasks

• Hide migrate_drupal_multilingual
• Add hook to uninstall it
• Add followup to remove module in Drupal 9.
• Review change record
• Answer questions in #22 and #23
• Commit it once #2208401: [META] Stabilise Migrate Drupal Multilingual module is done.

User interface changes

No more Migrate Drupal Multilingual module visible.

API changes

None. We keep the module for backwards compatibility.

Data model changes